The walking dead

There are examples of species all over the world that are essentially the walking dead," said Stanford University professor Paul Ehrlich. "We are sawing off the limb that we are sitting on."

He was talking about the Sixth Extinction, the huge loss of species that is underway right now. It has been discussed in public before, of course, but what Ehrlich and other scientists from Stanford and Princeton universities and the University of California Berkeley have done is to document it statistically.

Animals and plants are always going extinct, usually to be replaced by rival species that exploit the same ecological niche more efficiently. But the normal turnover rate is quite slow, according to the fossil record: about one species of vertebrate per 10,000 species goes extinct each century. Ehrlich and his colleagues deliberately raised the bar, assuming that the normal extinction rate is twice as high as that — and still got an alarming result.

In a study published this month in Science Advances, they report that vertebrates (animals with internal skeletons made of bone or cartilage — mammals, birds, reptiles and fish) are going extinct at a rate 114 times faster than normal. In a separate study last year, Professor Stuart Pimm of Duke University estimated that the loss rate may be as much as a thousand times higher than normal — and that includes plants as well as animals.

"We are now entering the sixth great mass extinction event," said Gerardo Ceballos of the Universidad Autonoma de Mexico, lead author of the Science Advances study. "If it is allowed to continue, life would take many millions of years to recover and our species itself would likely disappear early on." Indeed, Harvard biologist E.O. Wilson has estimated that at the current rate of loss, half of Earth's higher lifeforms will be extinct by 2100.

The previous five mass extinctions, all during the past half-billion years, each wiped out at least half of the existing species of life. Four of them were probably caused by drastic warming of the planet due to massive, millennia-long volcanic eruptions.

The warming eventually made the deep oceans oxygen-free, allowing sulfur bacteria to emerge from the muds. As they took over the oceans, they killed off all the oxygen-based life — and when they finally reached the surface, they emitted vast quantities of hydrogen sulfide gas that destroyed the ozone layer and directly poisoned most land-based life as well.

The fifth and most recent mass extinction, at the end of the Cretaceous era 65 million years ago, was different. It was caused by a giant asteroid that threw so much dust up when it hit Earth that the Sun was effectively hidden for years. First the plants died, and then the animals.

But the cause of the sixth extinction is a single species: us.

It's fair to say that we are the victims of our own success, but so is the entire biosphere. There were one billion of us in 1800. We are now seven and a half billion, on our way to 10 or 11 billion. We have appropriated the most biologically productive 40 per cent of the planet's land surface for our cities, farms and pastures, and there's not much room left for the other species.

They have been crowded out, hunted out, or poisoned by our chemical wastes. Their habitats have been destroyed. Even the oceans are being devastated as one commercial fish species after another is fished out. And still our population continues to grow, and our appetite for meat causes more land to be cleared to grow grain not for people, but for livestock.

All this even before global warming really gets underway and starts to take huge bites out of the ecosphere. We are on the Highway to Hell, and it's hard to see how we get off it.

In a way, climate change is the easiest part of the problem to fix, because all we have to do is stop burning fossil fuels and reform the way we farm to cut carbon dioxide emissions. More easily said than done, as the history of the past 30 years amply demonstrates, but certainly not impossible if we take the task seriously.

Maintaining the diversity of species (some of which we haven't even identified yet) that provide essential "ecosystem services" is going to be far harder, because the web of interdependence among apparently unrelated species is very complex. At the very least, however, it is clear that we must restore around a quarter of our agricultural land to its original "wild" state and cut back drastically on fishing.

It's far from clear that we can do that in time and still go on feeding all of the human population, but the alternative is worse. James Lovelock put it very bluntly in his book The Revenge of Gaia.

"If we continue business as usual, our species may never again enjoy the lush and verdant world we had only a hundred years ago," he wrote. "What is most in danger is civilisation; humans are tough enough for breeding pairs to survive....but if these huge changes do occur it seems likely that few of the teeming billions now alive will survive."

Gwynne Dyer is an independent journalist whose articles are published in 45 countries.